Other forms of rotary seal are known which achieve a double balance, that is to say a balance with respect to both internal and external pressures, or more precisely to achieve such a balance to a predetermined degree or proportion. An example of such a rotary seal is illustrated in FIG. 1 of the accompanying drawings which shows a pusher seal.
In FIG. 1 a seal face member 2 is secured to rotate with a shaft 4. A stationary annular seal 6 insert engages the seal face 3 of member 2 to provide a seal interface between diameters A and D. The seal insert is carried by a stationary seal assembly 8 that is split into two parts 10 and 12. The inner part 10 directly carries the insert 6. The outer part 12 is secured to a support structure not shown so that to the right of the assembly, as seen in the figure, there exists an internal pressure P1 extending to the inner, shaft, side of the seal interface and to the left there exists an external pressure P2 acting on the outer side of the seal interface. The two parts 10 and 12 define between them an annular chamber 14 in which an O-ring 16 provides a seal between the two parts. The parts also define passageways 18 and 20 respectively by which the pressures P1 and P2 have access to, and are sealed from one another by, the O-ring 16. The O-ring acts between the seal interface diameters B and C.
As regards the internal pressure P1 the seal interface is balanced from diameter D to diameter B; for the external pressure P2 the seal interface is balanced from diameter A to diameter C. In the case, for example, of what is known as a 70/30 doubly balanced seal, the respective seal face areas between A and B on the one hand and C and D on the other will each be 30% of the total area.